1. Field of the Invention
The present invention relates to optical glass, a precision press molding preform and manufacturing method of the same, an optical element and manufacturing method of the same. More particularly, the present invention relates to optical glass having prescribed optical constants and good precision press molding properties, a precision press molding preform and an element both comprised of the above optical glass, and methods for manufacturing the same.
2. Discussion of the Background
The advent of digital cameras and portable telephones equipped with cameras has led to rapid increases in the integration and functions of devices employing optical systems. Under such circumstances, the need for ever greater precision, weight reduction, and size reduction has been increasing in optical systems.
In recent years, to satisfy these requirements, optical designs employing aspherical lenses have been entering the mainstream. Thus, to stably and inexpensively supply large quantities of aspherical lenses comprised of high-functionality glass, precision press molding techniques (also called mold-forming techniques) by which optically functional surfaces are directly formed by press molding without a grinding or polishing step have attracted attention. The demands placed on optical glass having low-temperature softening properties suited to precision press molding are increasing each year. Such optical glasses include high refractive index, low dispersion glasses having a refractive index (nd) of equal to or greater than 1.65 and an Abbé number (νd) of 50 to 60. An example of a high refractive index, low dispersion glass having low-temperature softening properties is described in Japanese Patent No. 2,616,958 (Reference 1).
The following is conceivable in manufacturing optical elements such as lenses by precision press molding the above-described optical glasses. The glass described in Reference 1 has either an excessively high or excessively low content of SiO2, which greatly affects the viscosity of the glass. When the content of SiO2 is excessively low, viscosity varies sharply with temperature, the temperature range suited to precision press molding becomes extremely narrow, and glass molded in a pressing mold ends up cracking, compromising the yield during manufacturing. When the content of SiO2 is excessively high, the glass transition temperature (Tg) rises, which is disadvantageous in precision press molding. Accordingly, a large quantity of alkali metal oxide is incorporated into the glass described in Reference 1 to lower the glass transition temperature (Tg). However, when a large quantity of alkali metal oxide is introduced, the stability of the glass greatly deteriorates. It is extremely difficult to achieve both a low-temperature softening property and stability while preventing cracking during press molding in high refractive index, low dispersion glass in this manner.
In light of these problems, it is an object of the present invention to provide optical glass that can be used to manufacture optical elements by precision press molding with high productivity, a precision press molding preform comprised of this glass, a method for manufacturing the same, and a method for manufacturing an optical element in which an optical element is produced with high productivity using this preform.